Apparatus for operating electric discharge devices



Dec. 7, 1965 w. F. POWELL, JR 3, 22,572

APPARATUS FOR OPERATING ELECTRIC DISCHARGE DEVICES Filed July 23, 1962 3 Sheets-Sheet 1 L vvv'L MW BILATERAL SWITCH l3 R2 J l5 AAA- INVENTOR. WALTER F. POWELLJR.

BYZAV/Q $1M ATTORNEY Dec. 7, 1965 w. F. POWELL, JR

APPARATUS FOR OPERATING ELECTRIC DISCHARGE DEVICES 5 Sheets-Sheet 2 Filed July 23, 1962 vvvvv BILATERAL SWITCH llllll AAAA n INVENTOR.

4 Q A W BILATERAL 5 5 BILAT E RAL SWITCH SWITCH FEEDBACK CONTROL 56 AAAA WALTER F. POWELLJR BY 7%. W

ATTORNEY United States Patent This invention relates generaly to apparatus for operating electric discharge devices such as fluorescent lamps and more particularly, it relates to an improved apparatus for operating such devices.

It is an essential requirement of an apparatus used to operate electric discharge devices that the current supplied to the electric discharge device be regulated. Unless the current supplied to the electric discharge device is regulated, the lamp current in the lamp becomes excessive and the lamp will destroy itself because of its inherent negative resistance characteristic.

Heretofore, passive ballasting systems have been used to operate electric discharge devices such as fluorescent lamps or mercury vapor lamps. In such passive systems a ballasting means, usually a reactive element, is employed to achieve a balance between the energy available at the source and the energy required to operate the electric discharge lamp. In a commonly used ballasting system, the reactive element, the driving voltage source and the electric discharge lamp are associated in a series loop arrangement. As the lamp current builds up because of the negative resistance characteristic of the device, the current through the reactive element builds up causing an increase in the voltage drop across the reactive element. Consequently, the voltage drop across the lamp decreases since the sum of the voltage drops in the lamp circuit must be equal to the driving voltage which is fixed. As the voltage across the series reactor increases, the lamp current diminishes. In this manner, the ballasting element passively regulates the current supplied to the lamp to provide a ballasting action.

A disadvantage of a conventional passive ballast system is that the volt-ampere requirements of the reactive elements in the system must be relatively high and also the peak energy stored in the reactive elements must be relatively high in order that the ballasting action can be effected with satisfactory regulation and stability. To provide satisfactory regulation when the supply voltage varies plus or minus ten percent, it is necessary that the ballasting volt-ampere characteristic intersect the normal volt-ampere characteristic of the electric discharge lamp at a sufficiently steep angle. It will be understood that in passive ballast systems supply voltages two or three times as great as the lamp operating voltage are usually required to provide a satisfactory ballasting characteristic. It is desirable, therefore, to reduce the volt-ampere requirements of the ballast while achieving satisfactory regulation and stability.

Accordingly, it is a general object of the invention to provide an improved apparatus for operating one or more electric discharge lamps.

A more specific object of the invention is to provide an improved apparatus for operating electric discharge lamps that is readily adaptable to control by feedback means.

It is another object of the invention to provide an improved apparatus for operating electric discharge lamps wherein the volt-ampere requirements of the apparatus are not relatively high.

A still further object of the invention is to provide an improved apparatus for operating fluorescent lamps wtih relatively less peak energy stored in the reactive elements employed for ballasting.

In accordance wtih one aspect of my invention, I have provided an improved apparatus for operating one or 3,222,572 Patented Dec. 7, 1965 more electric discharge lamps in which a control means for varying the energy supplied from a power source dynamically achieves an approximate balance between the energy supplied by the power source and the energy required for operation of the electric discharge lamp or lamps by switching the supply energy from one predetermined level to another. A feedback circuit is coupled with control means to activate the control means and to vary the energy supplied to the lamp in response to a feedback signal. The feedback signal may be proportional to the lamp current or may have some other functional relationship to the current or other parameter.

In a form of my invention, a dynamic balance is achieved between the energy supplied at the input of the apparatus and the energy required for lamp operation by a switching arrangement in which a switching means is driven in response to a feedback signal to cause an impedance element to be effectively cut in and out of serial connection with the lamp and thereby regulate the energy supplied to the lamp.

According to another form of my invention, I provide a control means for varying the energy supplied from the source which includes a transformer in conjunction with a switching arrangement wherein a portion of the transformer secondary is repetitively switched in and out of the circuit to better dynamically balance the source energy with the energy required for lamp operation. The switching action is controlled by a feedback circuit which is activated in response to a feedback signal indicative of a lamp operating condition.

In accordance with my invention, the electric discharge of the lamp or lamps is stabilized by the dynamic control exercised by the apparatus over the source energy to provide the energy required by the lamp. Preferably, the amount of energy supplied to a lamp is controlled and regulated in response to changes in the parameters sensed or detected by the feedback circuit. By way of illustration, the feedback cricuit may sense such parameters as lamp current, light output, lamp voltage, lamp power, lamp arc stream temperature, or combinations of such parameters to provide a suitable feedback signal for controlling the operation of a lamp.

The subject matter which I regard as my invention is particularly pointed out and claimed in the concluding portion of this specification. My invention, however, both as to its structure and manner of operation, together with further objects and advantages thereof, may be best understood by reference to the following description taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a schematic illustration of one embodiment of the invention wherein the control means in accordance with the invention is driven in response to a feedback signal that is proportional to the lamp current;

FIGURE 2 is a schematic illustration corresponding to the embodiment of the invention shown in FIGURE 1 wherein the control means is driven in response to a feedback signal that is proportional to the light output of an electric discharge lamp;

FIGURE 3 is a schematic circuit diagram of the bilateral switch shown in block form in the illustration of FIGURE 1;

FIGURE 4 is a schematic circuit diagram of another embodiment of my invention wherein the electric discharge lamp is operated with a unidirectional current;

FIGURE 5 is a schematic illustration of an alternative embodiment of the invention for operating electric discharge lamps with an alternating current supply; and

FIGURE 6 is a schematic diagram of the bilateral switches and the feedback control shown in block form in the illustration of FIGURE 5.

Referring in more detail to the FIGURE 1 of the drawing, I have shown therein an apparatus, generally identified by reference numeral 10, for operating an electric discharge lamp 11. The components of the apparatus are shown enclosed in a dashed rectangle. A pair of input terminal leads 12, 13 are provided for connection to a suitable alternating current supply. The output of the apparatus 10 is applied to the electric dis charge lamp 11 by output leads 14 and 15.

In accordance with one form of the invention, a bilat eral switch 16 is driven in response to a feedback signal supplied at the feedback leads 17 and 18 connected across a shunt resistor R The bilateral switch 16 is repetitively switched to an open position (high impedance state) and a closed position (low impedance state) in each half cycle of the alternating current supply to dynamically balance the source energy with the energy required to operate the electric discharge lamp 11 connected across the output leads 14, 15. When the bilateral switch 16 is driven to the open position, an impedance element, such as the resistor R used in the illustrated exemplification of the invention, is effective in the lamp circuit. The voltage applied to lamp 11 does not change immediately since the voltage across the inductor L assumes a polarity that is additive with respect to the supply voltage. As energy is released from the inductor L and the current decreases toward a lower steady-state level, the lamp voltage momentarily increases. Essentially, the actuation of the bilateral swich 16 varies the impedance load in the circuit with the lamp voltage remaining nearly constant.

When lamp 11 is operated by the apparatus 10, the current is kept in continuous transition between two steadystate current levels, one of which by itself is too high and the other too low. An inductor L is provided to introduce a small amount of inductance in the circuit to control the rate at which the transition proceeds between the two steady-state levels. It will be appreciated that the inductor L is a relatively small high frequency inductor and would not provide sufficient reactance to control the operation of the electric discharge lamp 11 in accordance with the conventional practice. In the exemplification of the invention, the inductor L had an inductance of approximately 30 millihenries.

In the circuit diagram of FIGURE '2, I have illustrated an apparatus 20 wherein a bilateral switch 21 is activated in response to a feedback signal from a photoelectric cell 22 disposed near the electric discharge lamp 11 to sense the light output and regulate the energy supplied to lamp 11 in response to the variations in the light output. The feedback signal from the light sensitive cell 22 is supplied to the bilateral switch 16 by leads 23, 24.

The apparatus 20 as shown in FIGURE 2 is generally similar to the apparatus 10 of FIGURE 1. Accordingly, I have used the same reference numerals to identify the cor responding parts thereof. The shunting impedance element R is connected in shunt across the bilateral switch 21. To control the rate of change of the rapid excursions of current, a high frequency inductor L is connected in series with the bilateral switch 21.

The operation of the apparatus 10 shown in FIGURE 1 and the apparatus 20 as shown in FIGURE "2 will now be described. During each alternation of the power supply, the bilateral switches 16 and 21 are driven repetitively from a normally closed position to an open position in response to a feedback signal. The repetitive switching action causes the energy supplied at the output leads 14 and to be dynamically varied to control the operation of lamp 11. When the bilateral switch 16 or 21 is in its normally closed position or in the open position, energy is supplied from the power source to output leads 14, 15.

Taking an arbitrary positive half cycle when the voltage at input terminal lead 12 is positive and the bilateral switch is closed, the path of current flow is from input terminal lead 12, through the inductor L the bilateral switch, output lead 14, lamp 11, output lead 15 and input terminal lead 13. It will be noted that when the bilateral switch is driven to the open position the current from the power supply is diverted through the impedance element R causing the current to decrease to the lower steadystate value. Thus, in both switching modes energy is being continuously supplied to the electric discharge lamp 11 from the power source and is dynamically balanced to meet the ballasting and operational requirements of the electric discharge lamp 11. In the apparatus 10 of FIG- URE 1, the switching action is controlled in response to a feedback signal determined by lamp current. However, in apparatus 20 shown in FIGURE 2, the bilateral switch 21 is driven in response to a feedback signal provided by the light sensitive cell 22.

Bilateral switches of the type which may be used in the practice of this invention are fast acting switches, such as transistors or silicon controlled rectifiers, which can be activated at switching rates above several hundred switchings per second. Preferably, switching rates bebtween 1000 and 50,000 switchings per second may be employed in the practice of the invention.

Referring now to FIGURE 3, I have illustrated therein a schematic circuit diagram of the bilateral switch employed in the embodiment of the invention illustrated in FIGURE 1. The switching action is performed by a transistor switch Q, a power PNP transistor, that is driven by a bistable flip flop circuit controlled by a feedback signal applied at leads 17, 18.

The bilateral operation of the transistor switch Q is achieved by connecting the transistor switch Q in a bridge arrangement employing four diodes D D D and D It will be seen that during the positive alternation of the alternating current supply when the voltage at input terminal lead 12 is positive, diodes D and D are forward biased and the diodes D and D conduct current through the transistor switch Q During the negative alternation of the power supply diodes D and D are reversed biased, and a forward bias is applied across diodes D and D causing these latter two diodes to conduct current. Thus, during the negative alternation a path for current flow is provided from lead 14 through the diode D transistor switch Q and diode D to input terminal lead 12.

The bistable flip flop circuit includes a tunnel diode TD and control transistors Q and Q Base drive for transistors Q Q and Q and bias current for the tunnel diode TD is provided by connecting terminals 25 and 26 to a negative and positive control supply. Resistor R connected with terminal 25 limits the base drive current for transistor Q and resistor R sets the bias current for the tunnel diode flip flop. In order to reduce the turnoff time, resistors R and R are connected in circuit with the base electrodes of transistors Q and Q Diodes D and D connected to the base electrode of transistor switch Q prevent the voltage drop developed across the collector and emitter electrodes of transistor Q when transistor Q is switched on, from driving the base-emitter junction of the transistor switch Q to a conducting state. A Zener diode D connected in shunt across the collector and emitter of electrode of transistor switch Q protects the transistor against transient overvoltages.

Essentially, the switching frequency of the transistor switch Q is determined by the rate at which the tunnel diode TD is switched to a high voltage state and reset by the pulses induced across secondary winding S of a pulse transformer T The bias supplied to the tunnel diode TD through resistor R is just sufiicient to maintain the tunnel diode TD near its peak point value. When a negative pulse is induced across the secondary winding S tunnel diode TD is switched to its high voltage state. Capacitor C is connected in circuit with the secondary winding S in order to provide for DC. isolation and pulse coupling of the secondary winding S with the bistable flip flop circuit.

To provide the pulses required to activate the flip flop circuit from one state to another, a pair of tunnel dioxide circuits are used and alternately come into play as the supply voltage swings positive and negative. During the positive alternation the tunnel diode circuit, which includes steering diode D tunnel diode TD resistor R and primary winding P of pulse transformer T comes into play. In the negative half cycle, the companion tunnel diode circuit, which includes the steering diode D tunnel diode TD resistor R and the primary Winding P comes into play. The resistors R and R prevent ringing in the tunnel diode circuits, and the feedback resistor R limits the feedback current supplied to the tunnel diode circuits. Also, without the resistor R the tunnel diodes TD TD would assume the potential across leads 17 and 18 and a switching action would not occur.

It will be appreciated that the feedback leads 17 and 18 may be connected in circuit with various sensing elements which will provide a suitable feedback signal for controlling the switching action of the transistor switch Q For example, the input signal may be provided by a feedback circuit which senses such parameters as line current, lamp voltage, lamp power, line voltage, are temperature, lighting illumination levels or combinations of these parameters. Although in FIGURES 1 and 2 the feedback signal provides a control that is responsive to the lamp current and light output, it will be understood that the feedback means may sense other parameters and control the operation of the lamp in response thereto. Preferably, the bilateral switch 16 may be driven in response to a lamp current feedback in conjunction with other feedback signals such as signals indicative of the light output or device temperature.

Referring now to FIGURES the bilateral switch 16 will now be more fully explained. During the open circuit condition of the lamp 111, the voltage of the alternatnig current source is essentially applied across the output leads 14 and 15. The bilateral switch 16 will remain in a closed position until the electric discharge lamp 11 ignites.

It will be understood that a starting aid circuit may be used to aid in the starting of the electric discharge lamp 11. Where an auxiliary starting circuit is used, it is generally possible to start an electric discharge lamp at a lower potential level. In a well-known starting aid arrangement, a conductive plate, which is usually the lamp fixture, is disposed in proximity to the electric discharge lamp so that a starting aid potential is initially applied between a lamp electrode and the conductive plate to aid in initiating the electric discharge in the vicinity of the electrode. Although I have not shown such auxiliary starting aid arrangements in the embodiments of my invention, it will be readily understood that such auxiliary starting aid arrangements are readily adaptable for use in conjunction with the apparatus of the invention.

At the start of an arbitrary half cycle of the alternating power supply when lamp 11 is ionized and the voltage is positive and increasing, the current through the normally closed bilateral switch 16 and through the shunt resistor R begins to increase. The voltage across the shunt resistor R is such that the right end is positive with respect to the left end, and a current flows through the feedback resistor R of the bilateral switch 16. Since steering diode D is now biased in a forward direction, current flows in two loops, one of which includes the tunnel diode TD and the other of which includes the resistor R and the primary winding P When this feedback current builds up to the peak point value of tunnel diode TD tunnel diode TD switches to a high voltage state. The increased impedance of tunnel diode TD causes a diversion of current to the primary winding P and a negative pulse is induced in the secondary winding S of pulse transformer T A negative pulse across the secondary Winding S causes tunnel diode TD to switch to its high voltage state. When tunnel diode TD is switched to a high 1 and 3, the operation of 6 voltage state, base drive is applied to the base electrodes of transistors Q and Q and they are switched into conduction. When transistors Q and Q conduct, the base drive is removed from transistor switch Q and consequently, the transistor switch Q is turned off.

When transistor switch Q is turned off, current from the power supply now flows through the impedance element R thereby causing a decrease in the energy supplied at the output leads 14 and 15. As bilateral switch 16 is opened and closed, energy supplied to the lamp is alternately switched from one level to another level back and forth. The inductor L controls the rate of change of the energy as the switching action causes excursions of energy from one energy level to another.

When the impedance element R is effectively placed in the lamp circuit, this is accompanied by a decrease in lamp current and in the voltage drop across the shunt resistor R Consequently, the current through the feedback resistor R decreases, and when the voltage across the tunnel diode TD reaches the valley point value, the tunnel diode TD is reset to its low voltage state. As a result, current is now diverted from the primary winding P and this change in current produces a change in the flux linking the secondary winding S causing a positive pulse to be induced in the secondary winding S The positive pulse across secondary Winding S causes the tunnel diode TD to be reset to its low voltage state. At the instant tunnel diode TD is reset to the low voltage state, base drive is diverted from both transistors Q and Q;; to cause them to be switched off. Base drive is now applied to the base electrode of the transistor switch Q Consequently, transistor switch Q is driven into conduction, and bilateral switch 16 is now closed. This switching action continues during the positive half cycle so long as the current remains at a sufiicient level to activate the tunnel diode TD During the negative alternation of the alternating current supply, the bilateral switch 16 is closed at the start of the cycle since the voltage is insufi'icient to activate the tunnel diode TD An increasing negative current now flows in the loop which includes tunnel diode TD the primary winding P of pulse transformer T and the resistor R When the current reaches the peak point value of tunnel diode TD the tunnel diode TD is switched into a high voltage state. This causes a diversion of current to the primary winding P and a negative pulse is induced in the secondary winding S As in the positive half cycle, the negative pulse will switch on transistors Q and Q and cause the base drive to be diverted from the base electrode of the transistor switch Q Thus, the bilateral switch 16 is now turned off and the impedance element R is effective in the lamp circuit to provide a reduction in the energy level supplied to the lamp. This is accompanied by the decrease in the voltage drop across the shunt resistor R When the voltage across the shunt resistor R reaches a point where the voltage across the tunnel diode TD is at its valley point value, the tunnel diode TD is reset to its low voltage state. Current from the primary winding is now diverted to the tunnel diode TD and this change in the flow of current through the primary winding P produces a positive pulse across the secondary winding S As previously described, a positive pulse across the secondary winding S will cause the transistor switch Q to be switched on. This switching action repeats itself during the negative half cycle so long as the voltage across the shunt resistor R is maintained at a sufficient level to actuate the tunnel diode TD It will be apparent that during each alternation of the power supply the bilateral switch 16 is repetitively opened and closed in response to a feedback signal that is functionally related to the lamp current. The switching action will continue as long as the feedback signal is sufiicient to activate the tunnel diodes TD and TD The operation of the apparatus 20 shown in FIGURE 2 is essentially similar to the operation of apparatus ill) of FIGURE 1. The bilateral switch 21 is repetitively opened and closed in each half cycle to provide controlled excursions of energy from one predetermined level to another and thereby to provide the operating voltage required to maintain a substantially constant light output and also to regulate the amount of energy supplied to the lamp 11. As the energy level across the output leads 14, 15 of the apparatus 20 increases, the instantaneous light output of the lamp ill also increases and the feedback signal to the bilateral switch 21 increases. The increasing feedback signal will cause the bilateral switch 21 to switch to an open position. Also, when the light output instantaneously decreases, the feedback signal decreases, and bilateral switch 21 is driven to its closed position. It was found that a repetitive switching action regulated in response to a feedback signal produced by variations of the lamp light output will control the operation of the lamp lll. A principal advantage of such a feedback arrangement is that an electric discharge lamp can be operated at a substantially constant light output during its normal life.

In FIGURE 4, I have illustrated an embodiment of my invention in which an electric discharge lamp 11 is operated with a unidirectional or unipolar voltage output across output leads 31 and 32. A pair of input terminal leads 33, 34 are provided for connection to a suitable DC. power source (not shown). As in the other embodiment of the invention, the energy applied at the input leads 33, 34 is dynamically balanced by the repetitive action of the transistor switch Q to meet the operating requirements of lamp 11. As transistor switch Q; is repetitively driven from a normally closed position to an open position, the electric discharge lamp Ill is operated by alternately increasing and decreasing excursions of energy from the power source. The energy supplied to lamp 11 by these excursions is sufficient to operate the lamp.

A resistor R is connected across the base and emitter electrodes of the transistor switch Q; to reduce the turnoff time of the transistor switch Q and thereby increase its switching speed. A diode D is connected in circuit with the base electrode of the transistor switch Q, to prevent the voltage drop across the collector and emitter electrodes of transistor Q when transistor Q conducts from driving the base-emitter junction of transistor switch Q to a conducting state. The PNP transistor Q a tunnel diode TD a shunt resistor R and a feedback resistor R comprise a feedback controlled driver for the transistor switch Q Base drive for the transistor switch Q, is provided through a resistor R by connecting terminal 35 in circuit with a suitable negative DC. bias supply. A feedback resistor R limits the feedback signal supplied to the tunnel diode TD A terminal 36 is provided for connection to a positive bias source.

The transistor switch Q; is normally in a closed position because the tunnel diode TD, is normally in a low voltage state. It will be noted that when tunnel diode TD is in a low voltage state, the transistor Q is in a high impedance state, and base drive is applied from the negative bias supply through resistor R and diode D to the base electrode of the transistor switch Q A high frequency inductor L is provided to control the rate of current excursions resulting from the repetitive switching action of the transistor switch Q Operation of apparatus 30 is initiated by connecting the input terminal leads 33 and 34 to a negative and positive side of the direct current power supply, respectively. During the open circuit condition of the electric discharge lamp 11, the potential of the DC. source is essentially applied across the lamp 1].. When lamp 11 has ignited, the path of current flow is from the positive terminal 34, to the output terminal 32, the electric discharge lamp ]ll, output lead 31, the inductor L the shunt resistor R the transistor switch Q and to input terminal lead 33. As this current builds up, the voltage drop across the shunt resistor R also builds up, and when the feedback current through resistor R reaches the peak point value of the tunnel diode TD the tunnel diode TD switches to a high voltage state. Transistor Q is driven into conduction and removes the base drive from the transistor switch Q Transistor Q, is, therefore, switched to an open position or high impedance state. Impedance element R is now effective in the lamp circuit and causes the energy supplied to lamp 11 to decrease. Also, the voltage across the shunt resistor R decreases, and when the voltage across the tunnel diode TD reaches its valley point value, the tunnel diode TD is reset to a low voltage state. As a result, transistor Q is switched off and base drive is applied to the base electrode of the transistor switch Q to switch it to the closed position or low impedance state. Accordingly, the energy level supplied to the electric discharge lamp 11 now rises and the switching cycle repeats itself. The electric discharge lamp ill is operated with a varying DC. current controlled by a feedback signal determined by the lamp current.

Referring now to FIGURE 5, I have shown illustrated therein an embodiment of my invention generally identified by reference numeral 46, in which the output of a transformer T is dynamically switched to approximately match the operating requirements of a pair of lamps 1 and 2 by a pair of bilateral switches 41 and 42 driven by a feedback control 43 receiving a feedback signal from the lamp circuit. A pair of input terminal leads 46 and 47 are provided for connection with a suitable alternating current power supply. In apparatus 40, the input voltage is applied across a primary winding P of the transformer T A pair of secondary windings S and S are inductively coupled with the primary winding R, on a magnetic core 48. The transformer T must have a low leakage reactance and may preferably be bifilar wound. It will be noted that bilateral switch 42 is connected at one end to a tap 49 by electrical lead 50 and at the other end it is joined with the bilateral switch 41 by leads 51 and 52.

It will be seen that when the bilateral switch 42 is closed, bilateral switch 41 is synchronously opened. During this switching mode, only the voltage across the secondary winding S is applied across lamps 1 and 2. However, when the bilateral switch 42 is opened and bilateral switch 41 is closed, the voltage across the secondary windings S and S is applied across the lamps l, 2 and inductor L In essence, the bilateral switches 41 and 42 function as a single pole double throw switch to alternately connect the secondary winding S and the secondary windings S and S into the lamp circuit. The inductor L is a small high frequency inductor and provides a small voltage drop between the instantaneous secondary voltage of transformer T and the instantaneous lamp voltage.

Bilateral switches 41, 42 are connected in circuit with the feedback control 43 by means of the connecting leads 53, 54 and 55, 56. A feedback signal is supplied to the feedback control 43 by the electrical leads 57, 58 connected across a shunt resistor R Referring now to FIGURE 6, I have illustrated therein a schematic circuit diagram of the bilateral switches 41, 42 and the feedback control 43 corresponding to the blocks 41, 42 and 43 as shown in the diagram of FIGURE 5. Referring more specifically to the feedback control 43, it will be seen that a feedback signal is supplied to the feedback control 43 through leads 57 and 58, a feedback resistor R and to the steering diodes D and D Steering diode D is forward biased during the positive half cycle of the alternating current supply While diode D is forward biased during the negative half cycle.

It will be noted that primary Winding P of a pulse transformer T and a primary winding P of a pulse transformer T are connected in parallel with each other in a loop which includes a resistor R and a tunnel diode TD Similarly, primary windings P and P of the pulse transformers T T are connected in parallel with each other in a loop which include-s a resistor R and a tunnel diode TD Since the steering diode D conducts during the positive half cycle of the alternating current supply, the circuits which include tunnel diode TD and the primary windings P P are energized. In the negative half cycle, diode D conducts and allows current to flow to the circuits which include tunnel diode TD; and primary windings P and P The opening and the closing of the bilateral switches 41 and 42 are synchronized in each half cycle by the pulses generated across the secondary windings S and S A negative pulse across the secondary winding S will cause the normally closed bilateral switch 41 to be driven to an open position and a negative pulse across the secondary winding S will cause the normally open bilateral switch 42 to be activated to a closed position. The resistors R and R were provided to prevent ringing in the tunnel diode loops.

Referring now more specifically to the bilateral switches 41, 42 as shown in FIGURE 6, the schematic circuit diagrams of these switches will now be more fully described. The switching action of transistor switch Q of the bilateral switch 41 is controlled by a bistable flip flop circuit which is comprised of a PNP transistor Q and a tunnel diode TD7. The switching action of the transistor switch Q of bilateral switch 42 is controlled by a bistable flip flop circuit which includes the PNP transistors Q and Q and a tunnel diode TD A Zener diode D and the resistors R R control the bias conditions for tunnel diode TD Resistors R R and R limit base drive current to transistors Q Q and Q respectively. Similarly, Zener diode D and resistors R and R control the bias conditions for the tunnel diode T D of bilateral switch 41. Zener diodes D and D protect the transistor switches Q and Q against excessive transient voltages.

A capacitor C is connected in circuit with the secondary winding 8., of the pulse transformer T to provide for DC. isolation and pulse coupling of the secondary winding S., with the driver circuit. Also, a capacitor C is connected in circuit with the secondary winding S and performs the same function for the bilateral switch 42. Resistors R R and R reduce the turn-off time of the transistors Q Q and Q Diodes D D and D are connected respectively with the base electrodes of transistors Q Q and Q Diode D prevents the voltage developed across the collector and emitter electrodes of transistor Q when transistor Q conducts from driving the base emitter junction of transistor switch Q to a conducting state. Similarly, diode D prevents the voltage developed across the collector and emitter electrodes of transistor Q from driving transistor Q Diode D performs a similar function for transistor switch The bilateral switching action in bilateral switch 41 is provided by a bridge arrangement which includes the diodes D D D and D It will be seen that diodes D and D provide a path through transistor switch Q,- for current flow during the positive half cycle, and diodes D and D provide a path for current flow through the transistor switch Q during the negative half cycle. Similarly, in bilateral switch 42, diodes D and D provide a path through transistor Q during the positive half cycle and diodes D and D provide a path for current flow through transistor switch Q during the negative half cycle.

Having reference now to FIGURES 5 and 6, the operation of the apparatus 40 will now be more fully described. When the primary winding P of transformer T is initially energized, bilateral switch 41 is in a closed position and bilateral switch 42 is in an open position. Hence, the combined voltage across the secondary wind- 1Q ings S and S is applied across the electric discharge lamps 1, 2 and inductor L When lamps 1 and 2 are ignited, current flows in lamp circuit.

Assuming that the voltage across the shunt resistor R is such that the right end of the resistor R as seen in FIGURE 5, is positive, tunnel diode TD will be switched to a high voltage state when the current reaches the peak point value. At the instant tunnel diode "TD is switched, current is diverted to the primary windings P and P and a negative going pulse is induced in the secondary windings S and S As a consequence, tunnel diodes TD7 and TD, of the bilateral switches 41, 42 are switched to their high voltage state.

It will be noted that when tunnel diode TD'] is in a high voltage state, transistor Q is driven to conduction, and base drive is removed from transistor switch Q, to turn it off. Simultaneously, as tunnel diode TD of the bilateral switch 42 is switched into a high voltage state, transistor Q is switched on. As a consequence, base drive is removed from transistor Q, to turn it off and cause transistor switch Q, to be driven to conduction.

In this switching mode, as bilateral switch 41 is opened, bilateral switch 42 is synchronously closed. Thus, only the potential across the secondary winding S is essentially applied across lamps 1 and 2. It will be seen that the path of current flow during this switching mode is in a loop which includes the secondary winding S lead 50, bilateral switch 42, lead 52, the inductor L output lead 44, lamps 1 and 2, output lead 45 and the shunt resistor R The decreased potential applied across the lamps 1 and 2 results in a decline of the instantaneous lamp current.

When the voltage drop across the shunt resistor R drops to a point where the current through the tunnel diode TD reaches the valley point value, the tunnel diode is reset to its low voltage state, and new current is diverted from the primary windings P and P This change in current flow causes a positive pulse to be induced across the secondary windings S and S The positive pulse across the secondary windings S and S causes the tunnel diodes TD7 and TD respectively, to be reset to their low voltage state. Consequently, the bilateral switch 41 is restored to its normally closed position and bilateral switch 42 is synchronously restored to its normally open position.

During this switching mode, the path of current flow is in a loop which includes secondary windings S and S bilateral switch 41, lead 51, the inductor L output lead 44, lamps 1 and 2, output lead 45 and shunt resistor R The feedback signal supplied to the feedback control 44 is now increasing, and when this signal reaches the peak point value of tunnel diode TD the bilateral switch 41 is again switched to the open position and bilateral switch 42 is synchronously driven to a closed position. This switching cycle repeats itself during the positive half cycle of the alternating current supply so long as the voltage drop across the shunt resistor R is suflicient to drive the tunnel diodes TD and TD, to a high voltage state.

During the negative alternation of the power supply, the circuit loops which include tunnel diode TlD the primary windings P P and the resistor R come into play, and a negative voltage signal induced across the secondary windings S S will synchronize the switching action of the bilateral switches 41 and 42 in substantially the same manner as during the positive alternation. As bilateral switches 41 and 42 are synchronously switched from a closed to an open position, the energy supplied to the lamps 1, 2 is alternately varied between one level and another in response to the feedback signal. The transformer T and the driven bilateral switches 41, 42 serve as a control means to dynamically balance the energy at the source to provide the energy required for operation of the lamps l, 2 in response to the feedback signal supplied by the feedback control 43.

Referring now more particularly to the schematic circuit diagrams of the bilateral switches 41 and 42, as shown in FIGURE 6, the manner in which the bilateral switches 41 and 42 are synchronously driven will now be more fully explained. tI will be appreciated that since tunnel diode TD is normally in a low voltage state, transistor Q is turned off and base drive is supplied through resistor R diode D to the base electrode of the transistor switch Q Therefore, transistor switch Q, is normally in a closed position. Similarly, in bilateral switch 42 tunnel diode TD is normally in a low voltage state. As a result, transistor Q is turned off, and base drive is applied to the transistor Q through the resistor R and diode D When transistor Q; is turned on, base drive is diverted from the base electrode of the transistor switch Q and consequently, the transistor switch Q is normally in the open position.

Referring again to bilateral switch 41, at the instant a negative pulse is induced across the secondary winding 5,, tunnel diode TD7 is switched to its high voltage state causing base drive to be applied to the transistor Q Thereupon, transistor Q conducts and diverts base drive from the transistor switch Q Consequently, transistor switch Q; is switched to the open position. At the same instant that a negative pulse is induced across the secondary winding S a negative pulse is simultaneously induced across the secondary winding S of the pulse transformer T.,,. This negative pulse causes tunnel diode TD to be switched into a high voltage state. As a result, base drive is applied at the base electrode of transistor Q When transistor Q conducts, base drive is diverted from transistor Q As transistor Q, is switched off, base drive is applied to the transistor switch Q and transistor switch Q, is switched to a closed position.

Bilateral switch 41 will remain in an open position and bilateral switch 42 will remain in a closed position until the tunnel diodes TDq and TD,, are reset to their low voltage state. This occurs when a positive pulse is induced across the secondary windings S and S A positive pulse is induced across the secondary windings S, and S when one of the tunnel diodes TD TD is switched to its low voltage state. Thus, as the feedback signal applied at the feedback control 43 varies in response to the change in the lamp current, the apparatus 40 is repetitively shifted from one switching mode to the other. In this manner, the energy supplied to the lamps 1, 2 is regulated and maintained dynamically to provide the energy for operating the lamps 1, 2.

From the foregoing description of the apparatus of the invention and its operation, it will be apparent that the power source is not disconnected from the lamp circuit in any of the modes of operation but is controllably varied in response to a feedback signal to provide the energy required for operation of the lamps. An advantage of the apparatus of the invention is that it is possible to achieve an appreciable reduction in the volt-amperes required in the system as compared with conventional passive ballasting systems and to accomplish this end with good regulation and stability.

Where a conventional ballast, such as a reactor, is used to ballast an electric discharge lamp, a supply voltage about 200 or 300 percent above the lamp operating voltage is generally required to provide the required ballasting characteristic. For example, if we take a fluorescent lamp that requires an operating voltage of 40 volts r.m.s., this lamp can be operated from a 120 volt, 60 cycle power supply. Let us assume in this case that a series ballasting reactor would be used to provide a voltage drop of approximately 110 volts. If we assume that this fluorescent lamp draws a load current of l ampere, the volt-ampere requirement of the series ballasting reactor would be approximately 110 volt-amperes. However, with the dynamic energy balancing arrangement of the present invention, it is possible to maintain satisfactory stability in the fluorescent lamp by operating the lamp from a supply voltage of 40 volts r.m.s. by dynamically varying the voltage within 10 volts above or below the supply voltage. It will be understood, of course, in this instance that an auxiliary starting means would be provided to aid in starting the lamp. In such an apparatus the volt-ampere requirement would be about 10 volt-amperes. If the supply voltage varies as much as 10 percent, to provide a completely satisfactory operating point, the lamp may be operated from a 50 volt supply with a 20 volt variation above and below the supply voltage. In this case, the voltampere requirement would be 20 volt-amperes as contrasted with volt-amperes required by a conventional ballast.

It will be noted that in the aforedescribed embodiments of the invention, I have not shown any filatment transformers for supplying filament heating current for the lamps. If electric discharge lamps employing cathodes that require a continuous supply of heating current are employed, it will be understood that a filament transformer may be used to supply the necessary heating current to maintain the electron emission at the cathodes. Also, it will be appreciated that starting aid circuits may be used in conjunction with the improved operating circuit arrangement of my invention to aid in initially starting the electric discharge lamp.

An important advantage of the arrangement of the present invention resides in its adaptability to control by means of feedback. Conventional ballasting systems are passive and current is dependent upon the electrical impedance of the electric discharge lamp. As an electric discharge lamp ages, the light output of the lamp appreciably i iminishes. For example, when a conventional 60 cycle ballast is used to operate a fluorescent lamp, such as a 96 PG 17 power groove lamp, the light output of the lamp will diminish to approximately 70 percent of its original light output after 6000 hours of operation. With the present arrangement, it is possible to maintain the light output of electric discharge lamps at a substantially constant level during the normal life of the lamp. For example, if a light sensitive cell is used to provide a feedback signal to regulate the energy levels applied at the lamp, it is possible to operate the lamp in response to the light output of the fluorescent lamp and thereby dynamically balance the energy at the source with the energy required to operate the lamp at a substantially constant level for the normal life of the lamp. Although in the specific exemplifications of my invention I have used feedback arrangements sensing lamp current and the light output, it will be apparent that other quantities can be readily sensed by the feedback means to provide a basis for regulation and operation of the electric discharge lamp.

From the foregoing discussion of the advantages of the invention and the above description of the various exemplifications thereof, it is to be understood that many modifications may be made by those skilled in the art, without actually departing from the invention. It is, therefore, intended in the appended claims to cover all such equivalent variations that come within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An apparatus for operating an electric discharge lamp from a power source, said apparatus comprising: a power source, an electric discharge lamp, a control means for varying the energy supplied to the electric discharge lam p by the power source, a feedback circuit coupled to said lamp for providing a feedback signal indicative of a lamp operating condition and coupled with said control means to activate said control means in response to said feedback signal, and circuit means including input leads for connection to the power source and including output leads for connection with the electric discharge lamp to supply the output of the apparatus to the lamp, said circuit means connecting said control means in circuit with said output leads and said input leads, and said control means dynamically balancing the energy from the source with the energy required to control the operation of the electric discharge lamp by repetitively varying the energy supplied at said output leads from one predetermined level to another at a rate which varies as a function of the magnitude of current in said discharge lamp.

2. The apparatus set forth in claim 1 wherein said feedback circuit is connected in circuit with one of said output leads to supply a feedback signal to said control means that is functionally related to the lamp current.

3. An apparatus for operating at least one electric discharge lamp from a power source, said apparatus comprising: a power source, atl east one electric discharge lamp, a control means for repetitively varying the voltage across said electric discharge lamp from one predetermined level to another, feedback circuit for providing a feedback signal indicative of a lamp operating condition and coupled to said lamp coupled with said control means for activating said control means in response to a feedback signal indicative of a lamp operating condition, and circuit means including input leads for connection to the power source and including output leads for connection with at least one electric discharge lamp to supply the output of the apparatus to the lamp, said circuit means connecting said control means in circuit with at least one of said output leads and one of said input leads to place said control means in series circuit relation with the lamp, and said control means dynamically balancing the voltage at the source with the voltage required to control the operation of the electric discharge lamp by repetitively varying the energy supplied at the output leads from said one predetermined level to another in response to and at a rate indicative of the magnitude of said feedback signal.

4. An apparatus for operating at least one electric discharge lamp from an alternating power source, said apparatus comprising: an alternating power source, at least one electric discharge lamp at least one bilateral switch, means for repetitively driving said bilateral switch to a high impedance state and to a low impedance state in each half cycle of the alternating power source, an impedance element connected in shunt with said bilateral switch, circuit means including input leads for connection with the power source and output leads for connection with the electric discharge lamp to supply power from said power source to the lamp, said circuit means connecting said bilateral switch in circuit with at least one of said input leads and one of said output leads, said bilateral switch when driven from one of said impedance states to the other in each half cycle causing the energy supplied to said output leads to be changed from one level to a second level, said repetitive changing of said energy levels having a rate which varies as a function of the current drawn by said discharge lamp to dynamically balance the energy provided by said source with the energy required for operation of the electric discharge lamp.

5. A system for operating at least one electric discharge lamp from an alternating power source, said system comprising: an alternating power source, at least one electric discharge lamp, at least one bilateral switch, means for repetitively driving said bilateral switch to a high impedance state and to a low impedance state in each half cycle of the alternating power source, a first impedance element connected in shunt with said bilateral switch, an inductance connected in series circuit relation with said bilateral switch, circuit means including input leads for connection with the power source and output leads for connection with the electric discharge lamp to supply the output of the apparatus to the lamp, said circuit means connecting said bilateral switch in circuit with at least one of said input leads and one of said output leads, said bilateral switch when driven from one of said impedance states to the other in each half cycle causing the energy supplied to said output leads to be changed from one level to a second level, a feedback device coupled to said lamp for indicating the energy utilized by said lamp, and means coupling said feedback device to said driving means for repetitively changing said energy levels at a rate proportional to the energy utilized by said lamp to vary the impedance of said inductance and dynamically balance the energy provided by said source with the energy required for operation of the electric discharge lamp.

6. A system for operating an electric discharge lamp from a power source, said system comprising: a power source, an electric discharge lamp, a switching means, driver means for repetitively driving said switching means to a high impedance state and to a low impedance state, a feedback circuit coupled to said lamp and to said driver means to activate said driver means in response to a feedback signal, an impedance element connected in shunt with said switching means, circuit means including input leads for connecting with the power source and output leads for connection with the lamp to supply the output of the apparatus to the lamp, said circuit means providing a path for the supply of energy from the power source to said lamp when the switchingmeans is driven to a low impedance state and to provide a path for the supply of energy from the power source to the lamp through said impedance element when the switching means is driven to the high impedance state, said repetitive switching action of said switching means being at a rate responsive to said feedback signal to dynamically balance the energy at the source with the energy required for operation of the electric discharge lamp.

7. The system set forth in claim 6 wherein an inductive element is connected in series circuit relation with said switching means.

8. A system for operating an electric discharge lamp front an alternating supply, said system comprising: an alternating supply, an electric discharge lamp, a control means for controlling the power supplied to said electric discharge lamp, said control means including a bilateral switch, an impedance element connected in shunt therewith, a driver means coupled with said bilateral switch to repetitively activate said bilateral switch to a high impedance state and to a low impedance state during each half cycle of the alternating supply in response to a feedback signal that indicates the magnitude of power supplied to said lamp, said feedback signal controlling the switching rate of said switching means, circuit means including input leads for connection with the alternating supply and output leads for connection with the lamp to apply the output voltage across the lamp, said circuit means connecting said bilateral switch in circuit with one of said output leads and one of said input leads to provide a path for the supply of energy from the source to the lamp through said bilateral switch when said switch is in the low impedance state and toprovide a path from the source to the lamp through said impedance element when said bilateral switch is in the high impedance state.

9. The system set forth in claim 8 wherein an inductor is included in series circuit relation with said bilateral switch to control the supply of energy to the lamp as a functon of the switching time of said bilateral switch being activated from one state to the other.

10. A system for operating an electric lamp from an alternating power source, said system comprising: an electric lamp, a power source having a plurality of selectable voltages, control means including at least one transistor switch, circuit means including input leads for connection to said voltages of the power source and output leads for connection with the electric discharge lamp to supply operating voltage tothe lamp, said circuit means connecting said control means in circuit with one of said input leads and one of said output leads, said transistor switch being repetitively connected. to alternate input leads to cause the voltage applied across said output leads to instantaneously vary from one predetermined voltage level to another at a rate which varies as a function of the magnitude of current in said lamp, said control means thereby dynamically balancing the energy at the power source in each half cycle of the alternating power source with the energy required for operation of the lamp.

11. The system set forth in claim wherein a feedback circuit for providing a feedback signal indicative of a lamp operating condition is coupled to said lamp and to said voltage control means to activate said control means in response to a feedback signal.

12. An apparatus for operating an electric discharge lamp comprising: an alternating power source, an electric discharge lamp, a control means including a transistor switch and a driver means coupled to said transistor switch to control its switching action, circuit means including input leads for connection to the power source and including output leads for connection with the electric discharge lamp to supply the output of the apparatus thereto, said circuit means connecting said transistor switch in cincuit with one of said output leads and one of said input leads, a light sensing means coupled with said driver means to activate sad driver means to vary the impedance of said control means between first and second impedance levels in response to the light output of said lamp, said switching action of said transistor switch causing the current supplied at the output leads to vary from one current level to another, said control means switching between said first and second impedance level in each half cycle of the alternating power source to control the operation of the lamp.

13. The apparatus set forth in claim 12 wherein an inductor is included in series circuit relation with said transistor switch to control the rate of change of current supplied at the output leads between said current levels.

14. An apparatus for operating an electric discharge lamp, said apparatus comprising: a direct current source, an electric discharge lamp, input leads for connection to the direct current source, a pair of output leads for supplying the output of the apparatus to at least one electric discharge larrnp, a switching transistor having an emitter, collector and base electrode, said emitter and collector electrodes being connected in circuit with one of said input leads and one of said output leads, circuit means connecting the other of said output leads in circuit with the other of said input leads, a driver means coupled with said switching transistor to activate said switching transistor between a high and low impedance state to vary the impedance in circuit with the electric discharge lamp thereby causing the current supplied at the output leads to vary from one current level to another, said driver means being driven in response to a lamp operating condition and said switching transistor being repetitively activated to operate said at least one electric discharge lamp between said current levels at a rate which varies as a function of the magnitude of current in said lamp.

15. The apparatus set forth in claim 14 wherein an impedance element is connected in series circuit relation with said switching transistor to control the rate of change of the current between said current levels.

16. An apparatus for operating at least one electric discharge lamp from a power source, said apparatus comprising: a power source, at least one electric discharge lamp, a control means for repetitively varying the voltage across said electric discharge lamp from one predetermined level to another, a feedback circuit including a light sensing element disposed in proximity to said lamp to provide a feedback signal indicative of the operating condition of said lamp, means coupling said feedback circuit to said control means for activating said control means in response to said feedback signal and thereby activate said control means as a function of the light output of said electric discharge lamp, and circuit means including input leads for connection to the power source and including output leads for connection with at least one electric discharge lamp to supply the output of the apparatus to the lamp, said circuit means connecting said control means in circuit with at least one of said output leads and one of said input leads to place said control means in series circuit relation with the lamp, and said control means dynamically balancing the voltage at the source with the voltage required to control the operation of the electric discharge lamp by repetitively varying the energy supplied at the output leads from said one predetermined level to another in response to and at a rate indicative of the magnitude of said feedback signal. 17. An apparatus for operating at least one electric discharge lamp from an alternating power source, said apparatus comprising: an alternating current power source, at least one electric discharge lamp, at least one bilateral switch having an impedance element connected in shunt with said bilateral switch, means for repetitively driving said bilateral switch to a high impedance state and to a low impedance state, circuit means including input leads for connection with said power source and output leads for connection with said electric discharge lamp to supply power from said source to said lamp, said circuit means connecting said bilateral switch in circuit with at least one of said input leads and one of said output leads, said bilateral switch when driven from one of said impedance states to the other in each half cycle causing the energy supplied to said output leads to be changed fIOHI' one level to a second level, the repetitive changing of said energy levels having a rate which varies as a function of the current drawn by said discharge lamp to dynamically balance the energy provided by said source with the energy required for operation of the electric discharge lamp, and a feedback circuit operatively coupled to said lamp and to said means for repetitively driving said bilateral switch to activate said switch in response to variations in the light output of said electric discharge lamp.

References Cited by the Examiner UNITED STATES PATENTS Re. 24,671 7/1959 Jensen 323-22 2,411,440 11/1946 Le Page 315-151 2,779,897 1/1957 Ellis 315-156 2,872,623 2/1959 Bird 315 -224 3,146,392 8/1964 Sylvan 32322 3,156,860 11/1964 Paynter 32322 JOHN W. HUCKERT, Primary Examiner.

JAMES D. KALLAM, DAVID J. GALVIN, Examiners. 

5. A SYSTEM FOR OPERATING AT LEAST ONE ELECTRIC DISCHARGE LAMP FROM AN ALTERNATING POWER SOURCE, SAID SYSTEM COMPRISING: AN ALTERNATING POWER SOURCE, AT LEAST ONE ELECTRIC DISCHARGE LAMP, AT LEAST ONE BILATERAL SWITCH, MEANS FOR REPETITIVELY DRIVING SAID BILATERAL SWITCH TO A HIGH IMPEDANCE STATE AND TO A LOW IMPEDANCE STATE IN EACH HALF CYCLE OF THE ALTERNATING POWER SOURCE, A FIRST IMPEDANCE ELEMENT CONNECTED IN SHUNT WITH SAID BILATERAL SWITCH, AN INDUCTANCE CONNECTED IN SERIES CIRCUIT RELATION WITH SAID BILATERAL SWITCH, CIRCUIT MEANS INCLUDING INPUT LEADS FOR CONNECTION WITH THE POWER SOURCE AND OUTPUT LEADS FOR CONNECTION WITH THE ELECTRIC DISCHARGE LAMP TO SUPPLY THE OUTPUT OF THE APPARATUS TO THE LAMP, SAID CIRCUIT MEANS CONNECTING SAID BILATERAL SWITCH IN CIRCUIT WITH AT LEAST ONE OF SAID INPUT LEADS AND ONE OF SAID OUTPUT LEADS, SAID BILARERAL SWITCH WHEN DRIVEN FROM ONE OF SAID IMPEDANCE STATES TO THE OTHER IN EACH HALF CYCLE CAUSING THE ENERGY SUPPLIED TO SAID OUTPUT LEADS TO BE CHANGED FROM ONE LEVEL TO A SECOND LEVEL, A FEEDBACK DEVICE COUPLED TO SAID LAMP FOR INDICATING THE ENERGY UTILIZED BY SAID LAMP, AND MEANS COUPLING SAID FEEDBACK DEVICE TO SAID DRIVING MEANS FOR REPETITIVELY CHANGING SAID ENERGY LEVELS AT A RATE PROPORTIONAL TO THE ENERGY UTILIZED BY SAID LAMP TO VARY THE IMPEDANCE OF SAID INDUCTANCE AND DYNAMICALLY BALANCE THE ENERGY PROVIDED BY SAID SOURCE WITH THE ENERGY REQUIRED FOR OPERATION OF THE ELECTRIC DISCHARGE LAMP. 